The invention relates to a spark plug ignited internal combustion engine including a main combustion chamber and a separate ignition chamber communicating with the main combustion chamber. It is a disadvantage of internal combustion engines of the type described above that a fuel-air mixture, especially a lean fuel-air mixture, does not ignite readily, which results in non-uniform engine torque and, when the engine is relatively cool and/or is running with an enriched mixture, the walls of the chamber are subject to soot deposition. While these disadvantages which occur primarily in the partial load domain of the engine can be substantially reduced by raising the average temperature in the ignition chamber, this method brings the danger of thermal destruction or deterioration at high rpm and heavy loads because of the very high final temperatures in the ignition chamber which can even cause auto-ignition. It is known to limit the auto ignition of the fuel-air mixture in the ignition chamber by limiting the chamber temperature with engine cooling medium. However, such cooling must be dimensioned so as to take care of the full load operation at high engine speeds. Thus, while thermal destruction and auto ignition is avoided, substantially cooler temperatures obtain when the engine is operating at low power, for example while it is idling, at which time the susceptibility of the fuel-air mixture to ignition is substantially reduced. In addition, a cooled-off ignition point tends to cause sooting which further deteriorates the conditions for ignition and may cause a failure of ignition entirely.